Rest days

I ride about 5-6 days a week, so i usually got 1-2 rest days, but don't want to just sit at home and do nothing. What "other" physical activity is best for non-cycling days? Running, swimming, weights/gym?

Rest is good, but at the same time, so is a more diverse routine. If you are not concerned about mass, some lifting is always a good idea. Stay away from the legs, on rest days, unless you want to work in a light bike day sort of as a rest day from a hard leg workout. You can work it in, and would likely see some benefit.